1. Field of the Invention
The present invention relates to a method of and an apparatus for joining resin parts, and more particularly to a method of and apparatus for fusing resin parts or members such as the bumper beam and bumper face of an automobile bumper.
2. Description of the Relevant Art
Automobile bumpers generally comprise an outer bumper face and an inner bumper beam joined to an inner surface of the bumper face for reinforcing the same, the bumper is adapted to be mounted on an automobile body. The bumper face and beam have heretofore been heavy and liable to produce rust since they are made of pressed steel sheet.
Thermoplastic resin such as polypropylene or polyethylene is employed to make relatively large products inasmuch as it is lightweight and highly rigid. When automobile bumpers are constructed of such thermoplastic resin, they are lightweight and less apt to develop rust. For injection molding a relatively large resin component, it is general to form smaller divided members separately and then join the formed members subsequently into the resin component so that molds required are not increased in size and molded members can easily be removed from the molds. It is preferable to form an automobile bumper by molding a bumper beam and a bumper face separately. Various methods are available to join the bumper beam and face to each other. For example, they may be joined by adhesive, or interconnected by mechanical parts such as screws. Since they are made of thermoplastic resin, they may be joined by heating and fusing their joining surfaces with panel heaters held thereagainst and then pressing the fused joining surfaces to each other to connect the members integrally to each other.
The bumper beam is required to be higher in rigidity since it supports the bumper face, and hence should be reinforced by another member. A plastic composite material has been developed which is known as stampable resin sheet, which can be stamped to a desired shape. One known such stampable resin sheet is a glass fiber reinforced resin or plastic (hereinafter referred to as FRP) sheet comprising glass fibers impregnated with thermoplastic resin such as polypropylene or polyethylene. This sheet has a thickness ranging from 2 to 4 mm. It is preferable to form the above divided members by stamping a lamination of such sheets. The resin sheet thus formed is as rigid as steel sheet.
Before stamping a lamination of FRP sheets, each of the sheets must be fused by a heater or in a heated atmosphere. However, since the thermal conductivity of the resin is small, if the sheet were quickly heated, the surface layer thereof would reach a degradation temperature Td before the inside portion of the sheet would reach a melting temperature, resulting in degradation of the sheet. Therefore, it is current practice to heat an FRP sheet gradually in a relatively low-temperature atmosphere so that the temperature Ti' of the inside area of the sheet reaches a resin melting temperature Tm before the temperature Ts' of the surface layer of the sheet reaches a resin degradation temperature Td, as shown in FIG. 28 of the accompanying drawings. The graph of FIG. 28 has a horizontal axis representing heating time t and a vertical axis representing resin temperature Tp. It takes 4 minutes and 30 seconds to reach a heating completion time tc after a heating starting time t0. Accordingly, the efficiency of the heat treatment of the sheet is low, and so is the production rate for stamping the lamination of FRP sheets.
The processes for joining resin members using adhesive of mechanical parts are complex and have a low production efficiency. The process for joining resin members through fusion is more preferable.
Even where the latter fusion process is employed, however, it is somewhat difficult to have large joining areas of the bumper face and beam. Moreover, fused resin may be attached to the panel heaters, and foreign matter such as carbides may be trapped in the fused areas of the bumper face and beam, resulting in a reduced degree of bonding strength. One solution to these problems is to coat the surfaces of the panel heaters with Teflon. However, since Teflon layers are likely to peel off, the panel heaters have to be replaced periodically. In case the joining surfaces of the bumper beam and face are of different shapes, different heaters should be used for heating the jointing areas of different shapes. With an increased number of heaters used, the entire fusing apparatus is complex in structure. Furthermore, divided members formed by stamping FRP sheets contain fibers even at the joining surfaces, which become short of fusible resin and tend to develop insufficient joining forces.
The present invention has been made in view of the aforesaid problems of the convetional automobile bumpers, the process of stamping FRP sheets, and the process of fusing together bumper beam and face as resin parts.